In recent years, from the viewpoint of global environment conservation, the improvement of fuel efficiency of automobiles has been a critical issue. Development in which an increase in the strength of materials used for automobile bodies reduces thicknesses to lighten automobile bodies has been actively made. In general, however, an increase in the strength of steel sheets degrades formability. Thus, development of materials having high strength and high formability has been required.
TRIP steel utilizing transformation-induced plasticity of retained austenite has been receiving attention as a material satisfying such requirements. Various types of steel sheet have been developed to effectively utilize the effect. For example, Japanese Patent No. 2660644 discloses a steel sheet excellent in press formability by controlling the chemical composition and the amount of retained austenite in a steel sheet. Japanese Patent No. 2704350 discloses a method for producing such a steel sheet. Japanese Patent No. 3317303 discloses a steel sheet containing 5% or more retained austenite and having excellent formability (in particular, local ductility). Japanese Unexamined Patent Application Publication No. 2000-54072 discloses a steel sheet containing 3% or more retained austenite, having an average axial ratio of 3 to 20, and having an average hardness of a matrix of 270 HV or less and thus having a balance between stretch and stretch-flange formability.
Japanese Unexamined Patent Application Publication Nos. 2002-302734 and 2002-309334 each disclose a steel sheet containing 3% or more retained austenite and either 50% or more tempered martensite or 50% tempered bainite and thus having a balance between high ductility and high stretch-flange formability. Japanese Unexamined Patent Application Publication No. 2001-254138 discloses a steel sheet having an appropriate volume fraction of retained austenite, an appropriate content of carbon, and an appropriate aspect ratio in a ferrite phase and thus having excellent formability after preworking, and a method for producing the same.
Japanese Unexamined Patent Application Publication No. 2004-256836 discloses a high-tensile-strength hot-dip galvanized steel sheet having a sufficient strength-elongation balance and excellent fatigue properties and having a content of retained austenite of 3% or more, wherein 70% or more of grains of retained austenite has a ratio of the major axis to the minor axis of 0.2 to 0.4, i.e., an aspect ratio of 2.5 to 5. Japanese Unexamined Patent Application Publication No. 2004-292891 discloses a steel sheet also having excellent hole expansibility obtained by adjusting the steel sheet disclosed in JP '836 in such a manner that the proportion of martensite in a low-temperature transformation phase is 20% or less and that the ratio of the hardness of bainite in low-temperature transformation phase to the hardness of ferrite as a main phase is 2.6 or less.
However, the above-described known art has problems described below. In the steel sheet disclosed in each of JP '644 and JP '350, although sufficient ductility is obtained by utilizing the TRIP effect, stretch-flange formability is inferior to that of ferrite-martensite dual-phase steel. In the steel sheet disclosed in JP '303, local elongation is improved because strain-induced transformation does not easily occur even in a high strain region. However, strain-induced transformation occurs in a portion, such as a punched end face, subjected to high deformation, thus resulting in a small effect of improving stretch-flange formability after that. The steel sheet disclosed in JP '072 needs to contain 3% or more retained austenite and has an average axial ratio of 3 to 20. To form a lath shape having an average axial ratio of 3 or more, it is necessary to sufficiently promote bainite transformation during a final heat treatment step. That is, it is necessary to prolong an austempering time during the final heat treatment step. However, the austempering time is difficult to ensure in the known hot-dip galvanizing line in particular. Therefore, it is necessary to modify the process, e.g., a reduction in line speed, thereby reducing productivity.
In the steel sheet disclosed in each of JP '734 and JP '334, it is necessary to contain 3% or more retained austenite and either 50% or more tempered martensite or 50% or more tempered bainite. When a bainite structure or a martensite structure is used as a pre-structure, it is necessary to perform heat treatment under conditions different from usual conditions during a hot-rolling step or to perform continuous annealing twice. In the case where the bainite or martensite structure is formed during the hot-rolling step, the resulting steel sheet has high strength after the hot-rolling step. This increases rolling force during cold rolling, thus restricting a line. When the annealing step is repeated twice, production costs are markedly increased.
The steel sheet disclosed in JP '138 needs to have an appropriate volume fraction of retained austenite, an appropriate content of carbon, and an appropriate aspect ratio in a ferrite phase. To increase the aspect ratio and the carbon content, it is necessary to perform austempering, in which the steel sheet is held for a relatively prolonged period of time in a bainite-transformation-temperature range. Thus, in a hot-dip galvanizing line that is not capable of ensuring a sufficient austempering time after annealing, it is necessary to modify the process, e.g., a reduction in line speed, thereby significantly reducing productivity. In the steel sheet disclosed in each of JP '836 and JP '891, a structure before final annealing needs to be a structure including a low-temperature transformation phase such as bainite or martensite. Such a structure needs to be formed during a hot-rolling step or by repeating an annealing step twice. Providing such a step restricts a production line and increases production costs, as described above.
It could be helpful to provide a high-strength hot-dip galvanized steel sheet having excellent formability, the steel sheet eliminating special pre-structure control and capable of being produced by using a hot-dip galvanized steel-sheet production line that is not capable of sufficiently ensuring an austempering time after annealing, and to provide a method for producing the high-strength hot-dip galvanized steel sheet.